Organic covalent modification to improve thermoelectric properties of TaS2

Organic semiconductors are attracting considerable attention as a new thermoelectric material because of their molecular diversity, non-toxicity and easy processing. The side chains which are introduced into two-dimensional (2D) transition metal dichalcogenides (TMDs) by covalent modification lead to a significant decrease in their thermal conductivity. Here, we describe a simple approach to preparing the side chains covalent modification TaS2 (SCCM-TaS2) organic/inorganic hybrid structures, which is a homogeneous and non-destructive technique that does not depend on defects and boundaries. Electrical conductivity of 3,401 S cm−1 and a power factor of 0.34 mW m−1 K−2 are obtained for a hybrid material of SCCM-TaS2, with an in-plane thermal conductivity of 4.0 W m−1 K−1, which is 7 times smaller than the thermal conductivity of the pristine TaS2 crystal. The power factor and low thermal conductivity contribute to a thermoelectric figure of merit (ZT) of ~0.04 at 443 K.


Preparation of TaS 2 bulk single crystal:
2H-TaS 2 was synthesized by heating stoichiometric ratios elements of Ta (99.99%, Alfa Aesar) and S (99.999%, Alfa Aesar) in an evacuated quartz tube at 900 o C for several days, followed by slowly cooling the reaction quartz tube and its contents. The single crystal of 2H-TaS 2 were grown by the standard chemical vapor transport method.

IR spectroscopy of tert-Butyl isocyanate and SCCM-TaS 2 hybrid structure:
The IR spectra of tert-Butyl isocyanate and SCCM-TaS 2 hybrid structure bring the first evidence of covalent modification of the functional groups. We can preliminarily determine the reaction sites of the reactants from the IR spectra. The stretching vibration of (-N=C=O) band in 2280 cm -1 -2200 cm -1 is very strong in tert-Butyl isocyanate. Compared with the IR spectra of tert-Butyl isocyanate, the peak (-N=C=O) disappeared in the SCCM-TaS 2 hybrid structure, indicating that the chemical bond (-N=C=O) was broken. The S-C stretching is identified at ~ 650 cm -1 in SCCM-TaS 2 hybrid structure. It is confirmed that the organic covalent modification site is on the carbon atoms of N = C = O functional group.

X-ray photoelectron spectroscopy (XPS) on the pristine TaS 2 and SCCM-TaS 2 hybrid structure:
There is no Li peak in Supplementary Figure 6

Electrical conductivity and Seebeck coefficient measurement:
The electrical conductivity was measured in vacuum condition based on the four-probe method to exclude the effects of electrical contact resistance. Before the measurement, the sample was mechanically exfoliated into thin sheets (d~100 nm) and transferred to substrates. Parallel gold (d~100 nm) was evaporated as the electrode. The apparatus is a standard system for the measurement of Seebeck coefficient. The apparatus we used include Agilent B 1500A and a vacuum chamber.
The thermal voltage was measured by Agilent B 1500A in vacuum.

Hall measurement system:
The carrier concentration and mobility were measured in vacuum condition based on the Van der Pauw method. The measurement was carried out using the commercial PPMS measurement system (Quantum Design Inc. PPMS DynaCool-9T).

Fabrication of suspended microelectrodes:
The electrodes were fabricated by standard electron-beam lithography which provided by the Institute of Engineering Thermophysics, Chinese Academy of Sciences. In order to obtain the in-plane thermal conductivity of the pristine TaS 2 and SCCM-TaS 2 hybrid structure, in this measurement, the sample was transferred to the suspended microelectrodes by PMMA, and tested in vacuum. Figure 9. SEM image of sample on the suspended microelectrodes (a) and the thickness of mechanically exfoliated samples (b).

Thermal conductivity measurement:
Measurement system included lock-in amplifier 1, preamplifier 2, differential amplifier 3, adjustable resistor 4 and measuring terminal 5. The measuring method is conducted by using microelectrode #1 for heating and microelectrode #2 for measuring the temperature rise of the heat conduction caused by the heat flow through the sample. Microelectrode #1 is connected to a lock-in amplifier, and this lock-in amplifier is used to heat the microelectrode and measure the temperature rise of microelectrode #1. Microelectrode #2 is also connected to the other lock-in amplifier to measure the temperature rise of the heat receiving end. Using AC power at the same frequency can reduce the environmental interference. This measurement system has been calibrated by a standard platinum wire with a diameter of 20 μm placed on the suspended electrode, and its uncertainty is about 3.2%. This measurement process was carried out in room temperature and high vacuum environment (less than10 -4 Pa).
Supplementary Figure 10. The measurement system of thermal conductivity.